Organohalosilanes, particularly dialkyldichlorosilanes, are important intermediates in the silicone industry. The organohalosilanes are typically hydrolyzed and condensed to form polyorganosiloxanes that can then be processed to form, for example, silicone fluids, elastomers, sealants, adhesives, and resins. The predominant commercial process for preparing these organohalosilane intermediates is one commonly referred to as the “direct process” as originally described by Rochow in U.S. Pat. No. 2,380,995 (Aug. 7, 1945) and U.S. Pat. No. 2,380,996 (Aug. 7, 1945), which are incorporated herein by reference.
Because of the high volume of organohalosilanes used in the silicone industry, considerable effort has been devoted to optimizing the conversion of the silicon metalloid to the diorganodihalosilane, particularly to dimethyldichlorosilane. It is known in the silicone industry that different lots of chemical grade silicon metalloid react differently in the direct process. To attempt to control the variability of reactivity between lots of chemical grade silicon metalloid in the direct process, manufacturers of organohalosilanes have set strict controls on the acceptable types and levels of contaminants present in the silicon. For example, reference may be had to Volume 376 of the Journal of Organometallic Chemistry, by Michael P. Clarke, Pages 165-222, Nov. 7, 1989, entitled The Direct Synthesis of Methylchlorosilanes, which provides a comprehensive review of the direct process for synthesis of methylchlorosilanes and the effects of contaminants on the process.
U.S. Pat. No. 5,427,952 (Jun. 27, 1995) teaches a method for analyzing chemical grade silicon intended for use in the direct process for the presence of nonmetallic contaminants, including oxides and carbides of calcium, aluminum, and silicon. It is believed that a significant cause of the lot-to-lot variability of chemical grade silicon metalloid in the direct process is the presence of these nonmetallic contaminants. The method involves the separation of the contaminants by an alloying process and subsequent analysis of the contaminants for chemical composition.
U.S. Pat. No. 5,973,177 (Oct. 26, 1999), hereinafter the '177 patent, teaches a method for analyzing chemical grade silicon metalloid for oxide-impurities detrimental to the production of diorganodihalosilanes in the direct process by heating a sample under controlled conditions to a temperature greater than about 2300° C. in the presence of a carbon source such that a reduction product consisting of carbon monoxide and carbon dioxide which forms at above a temperature of about 1900° C. is predictive of chemical grade silicon metalloid performance in the direct process. In particular, the '177 patent teaches that the smaller the amount of reduction product formed above a temperature of about 1900° C., the greater is the specificity of the chemical grade silicon metalloid for the production yield of diorganodihalosilanes.
It has been found herein that the amount of detrimental oxide impurities present in chemical grade silicon metalloid can be predicted based on standard elemental analysis of the silicon metalloid and operating conditions of the refining process.